The neurotrophin, brain derived neurotrophic factor (BDNF), plays a critical role in brain development and function by regulating neuron survival and synaptic plasticity. Defects in BDNF signaling may lead to developmental, neurodegenerative and psychiatric disorders. Cellular responses to BDNF are mediated by the tyrosine receptor kinase, trkB, and their magnitude is determined by the relative levels of active, full-length trkB, and of truncated trkB isoforms, which act as dominant-negative inhibitors of BDNF signaling. Preliminary studies in cortical neurons revealed that expression of full-length trkB, but not that of truncated trkB, is stimulated by membrane depolarization via increased levels of intracellular Ca2+. Ca2+ was also found to exert opposing effects on the two promoters of the trkB gene (TRKB), suggesting that promoter usage regulates trkB isoform expression. These observations have led to the working hypothesis: TrkB isoform expression and consequently, neuronal responsiveness to BDNF, is regulated by Ca2+-dependent TRKB transcription. The mechanism by which Ca2+ differentially regulates the TRKB promoters will be studied using recombinant DNA technology to identify the Ca2+-dependent regulatory elements in TRKB and the transcription factors hat interact with them. The role of Ca2+-mediated promoter utilization in regulating the differential expression f full-length and truncated trkB will be studied by measuring the depolarization-induced expression f full-length and truncated trkB mRNA with promoter-specific 5'-untranslated sequences. In order to place Ca2+-dependent TRKB regulation in a more physiological context, we will investigate the ability of patterns of electrically evoked Ca2+ transients, which mimic in vivo neuronal activity, to differentially activate the TRKB promoters and differentially drive expression of full-length and truncated trkB isoforms. The long-term goal of this research is to elucidate the mechanisms by which Ca2+ regulates expression of trkB isoforms and, thus, modulates neuronal responsiveness to BDNF in the developing, adult and diseased brain.